1. Field of the invention
The present invention relates to a sewage treatment system or equipment for treating sewage water such as industrial effluent, various home life waste waters, etc. and, more particularly, to a sewage treatment equipment for oxidatively decomposing and eliminating water pollution materials such as BOD, COD by causing seawage water to contact, in the presence of air, an activated sludge process bed in which micro-organism (i.e., activated sludge) is placed on a carrier (i.e., fixed bed) and cultivated therein.
2. Description of prior art
With respect to the sewage treatment for oxidatively descomposing and eliminating water pollution materials such as BOD, COD contained in sewage water discharged from factories, hotels, homes and the like, the applicant has already proposed a sewage treatment equipment with activated sludge process beds in which a plurality of treatment tanks are installed in a row each communicating with an adjacent tank through a channel, and each of the treatment tanks comprises a plurality of activated sludge process beds disposed vertically above an aeration pipe connected to an air in-flow pipe and disposed at the bottom of each sewage treatment tank, each of the activated sludge process bed comprising a cylindrical core of which mesh-like or grid-like outer perphery is wrapped with a porous member of a certain thickness and tightened by tightening means, as is disclosed in the Japanese Patent Piblication (examined) No. 60316/1989. In this prior sewage treatment equipment, area of the mentioned outer periphery of the cylindical core wrapped with the porous member in each of the activated sludge process beds is increased tank by tank according to disposition of the treatment tanks so as to give variety to kind of bacteria, breeding ratio between aerobic and anaerobic bacterias and distribution thereof tank by tank in the order of instllation of the tanks.
In the sewage treatment equipment of above arrangement, when some air is supplied to the aeration pipe disposed at the bottom of each sewage tratment tank through the air in-flow pipe, the air is blown out into the sewage water in a form of bubbles through a lot of slits formed on the surface of the aeration pipe, and the sewage water containing oxigen of the air dissolved thereinto comes in contact with the activated sludge process beds above the aeration pipe. Thus, the water pollution materials such as BOD, COD are decomposed by aerobic and anarobic bacteria both implanted and cultivated on the activated sludge process beds. Meanwhile, these aerobic and anaerobic bacterias are further bred utilizing a part of energy obtained at the time of decomposition. In this connection, since each of the activated sludge process beds is cylindrical, there is a difference of dissolved oxygen concentration (hereinafter refferred to as DO value) between outside and inside of the cylinder, and aerobic bacteria is bred a lot on the outer peripheral surface of the bed, while anaerobic bacteria is bred on the inner peripheral surface of the bed. Then, giant micro-organism such as Zooglea bacteria, sphaerotilus, Nematoda or water mite are spontaneously bred or come to live at the center portion of the porous member of the activated sludge process bed and eat away both aerobic and anaerobic bacterias to autolize them. Therefore, excessive increase or breeding of aerobic bacteria to produce excessive sludge is successfully prevanted. Since a plurality of sewage treatment tanks are installed in a row, one tank being communicatted with the other tank adjacent through a channel, when DO value is changed by reducing the air supply from the aeration pipe to each tank in the order of installation of the tanks, there is a variation tank by tank in the aspects of kind of bacteria, ratio of aerobic to anaerobic bacteria, distribution of giant micro-organism, etc. cultivated in each sewage treatment bed. Thus, it becomes possible to utilize characteristics of respective micro-organism according to process of treatment, i.e., excellent performance of aerobic exhibited in the treatment of BOD, COD, etc. being at high load and that of anaerobic bacteria in the treatment of BOD, etc. being at low load as well as in the decomposition of nitrogen.
The foregoing improvement in the treatment efficiency depends largely on accurate control for achieving appropriate DO value. From this point of view, in the case of each activated sludge process beds used in the sewage treatment equipment according to the present invention, variety is given to the flow rate of the sewage water of which DO value is increased depending on the outside or inside of the cylinder by changing physical environment of the bed, i.e., cylindrical area wrapped with the porous member tank by tank in order. As a result, the control of BO value is quite accurate and easy both inside and outside of the cylindrical bed, and it is possible to control DO value appropriately keeping the nutritive balance in respective tanks disposed in a row, and accordingly the most suitable breeding phase of bacteria is constantly formed according to each process of the treatment. Thus, stable and efficient sewage treatment is constantly performed for long.
When employing the sewage treatment equipment of above arrangement disclosed in the Japanese Patent Publication No. 60316/1989, various advantages are assured such that even when the amount of air supply from the aeration pipe is kept as it was set at the starting of operation, a quite efficient sewage treatment is performed by the treatment equipment as a whole in spite of variation in BOD and COD in the sewage water, and it is possible to treat the sewage water at high efficiency at all times. Besides, the day-to-day operation control is easy and it does not require much labor.
However, in the mentioned sewage treatment equipment disclosed in the Japanese Patent Publication No. 60316/1989, sewage water simply flows into the first tank filling it and further flows into the next tank while overflowing from the first tank in such a manner as to flow slowly through every treatment tank with the in-flow air supplied by the aeration pipe. Therefore, a disadvantage may arise such that conditions of micro-organism membrance formed on the actvated sludge process beds are variable between upper region and bottom region of each bed, between center region and periphery region, or depending upon position of the disposed bed in a tank. To meet such a disadvantage, the prior equipment is controlled so as to give variety tank by tank to kind of bacteria, ratio of aerobic bacteria to anaerobic bacteria, etc. composing the activated sludge. However, there still remains a problem of not forming a particular kind of activatd sludge actually required by each tank depending upon portion of the bed in a tank. Accordingly, treatment efficiency of the entire tank is declined and, as a result, the expected sewage treatment performance of the entire equipment from high load treatment to low load treatment is not always satisfiably achieved.
Further, in the known sewage treatment of above arrangement, it is sometimes the case that no satisfiable sewage treatment is achieved due to seasonal change or climate pertinent to a district where the equipment is installed and operated. Furtheremore, when employing the mentioned prior sewage treatment equipment, it is certain that excess sludge is reduced remarkably as compared with the conventional activated sludge process. But in the high load treatment for treating an untreated sewage of which concentration of water pollution material mounts to 300 to 3,000 ppm, for example, a small amount of excess sludge is still produced even if using the mentioned equipment. Accordingly, it is required for such excess sludge to be regularly discharged out of the equipment, and it is a matter of course that additional cost is needed for such treatment of excess sludge. Moreover, when employing the prior sewage treatment of above arrangement, it is certain that water pollution materials including BOD, COD, etc. at high removing rate, but perfect decoloring of the treated water is difficult.